Means for irradiating materials

ABSTRACT

In electron irradiation of organic materials to cause chemical changes therein the material is subjected to at least five successive doses of electron radiation of substantially equal intensity, the material preferably being moved successively past at least five beams of electrons spaced apart along the direction of movement of the material.

United States Patent Norman Aug. 8, 1972 541 MEANS FOR mnmumuc MATERIALS[72] inventor: John I. Norman, Hinxton Hall, Safv fron Walden, Essex,England [22] Filed: March 11, 1970 [2]] Appl. No.: 23,524

[52] u.s.c| ..2so/49.s 11:,250/52 [51 int. Cl. .1101; 31/00 [58] Fieldof Search ..2so/49.s re, 52;:13/14 [5 6] References Cited UNITED STATESPATENTS Knowlton ..250I49.5 TE

Colvin ..250/49.5 TE Marker.... ..3l3/74 Primary Examiner-James W.Lawrence Assistant Examiner-C. E. Church Attorney-Scrivener, Parker,Scrivener and Clarke 1 [57] ABSTRACT in electron irradiation of organicmaterials to cause chemical changes therein the material is subjected toat least five successive doses of electron radiation of substantiallyequal intensity, the material preferably being moved successively pastat least five beams of electrons spaced apart along the direction ofmovement of the material.

1 Claim, 3 Drawing Figures PATENTED AUG 8 1972 saw 1 or 2 cgckcmuceuBpfi m to: om

MEANS FOR IRRADIATING MATERIALS This invention relates to an improvedmethod of, and apparatus for, continuous treatment of certain organicmaterials to cause beneficial changes in them by the action of electronirradiation.

Many of the organic materials and mixtures of organic materials(including mixtures with inorganic substances) which are known toundergo beneficial changes on irradiation exhibit an overall dose rateeffect such that the properties of the irradiated material are dependentnot only on the total irradiation dose received (in megarads) but alsoon the rateat which it is delivered. This phenomenon has been widelystudied using substantially continuous and uniform sources ofirradiation such as gamma rays and lowintensity electron beams, and ithas been shown that for classical systems which have a simplebimolecular termination the dose required to achieve a given degree ofreaction is proportional to the square root of the steady doserate.

This general phenomenon is exhibited by most freeradical polymerizationsof monomers including those that take place in the presence of polymers,to give graft polymerization. Specifically it is observed for systems ofunsaturated polymers (such as unsaturated polyesters) blended withunsaturated monomers (such as styrene). These systems can be applied asliquid films to manufactured materials and then polymerized in situ byelectron radiation to give hard and durable coatings, and theradiation-curing process has many advantages over conventional thermalcuring processes, particularly for high linear processing speeds.However for high linear speeds, conventional electron beam sourcesoperate at very high dose rates, such that the dose required for doserate dependent systems may be uneconomical or so high as to generateunacceptable temperatures.

However many systems and materials which do not exhibit a true orclassical type of dose-rate, do exhibit an overall dose-rate effect fora variety of reasons. We are only here concerned with those for which areduction in dose-rate is desirable and in most such cases, the cause ofthe overall dose-rate dependence is the heat produced by the adsorptionof energy from the electrons (together in some cases with the heat ofthe reaction initiated by the irradiation).

The heat produced is directly proportional to the dose absorbed, but thetemperature to which the material is raised also depends upon the rateof loss of heat and so is dependent upon the dose-rate, or overall timeof the irradiation process. As it is the temperature (or product oftemperature and time) which determines the reactions or processes, suchsystems have a definite dose-rate dependence. The form of such adependence is usually such as effectively to set a limit on the steadydose-rate which can be used, rather than a relatively continuousrelationship between dose and dose-rate.

It is thus seen that it is desirable to spread the exposure over aconsiderable distance in the direction of travel. However, to produce abeam of electrons uniformly covering an appreciable area (i.e. the widthof the material by the length) which may be several square feet,involves serious difficulties in electron optics and in the provision of-a suitable window for the electrons. Particularly at lower voltages(e.g. below 300 kilovolts) large windows which are thin enough totransmit the electrons efficiently would be very vulnerable and so wouldrequire a large pumping capacity to handle leaks.

An object of the invention is to provide a novel method of irradiatingmaterials in an economical manner with electrons of relatively lowenergy and at low dose-rates.

A further object of the invention is to provide apparatus suitable forcarrying out this method.

We have now found that the total radiation dose required to achieve agiven desired property (e.g. degree of polymerization, or cross-linking)for a given chemical system which exhibits adose-rate dependance" may besignificantly reduced by a new method of processing and/or a new designof electron source. While the main advantages in reducing the total doseare economic (lower power consumption and capital cost) there are alsoother advantages, i.e. less shielding is required and less secondaryheat is developed in the material being irradiated.

According to the invention we now propose to provide electronirradiation equipment consisting of a number of electron beams spacedapart along the direction of movement of the material to be processed.The dose required to achieve a given degree of change in the materialbeing treated depends upon the number of separate beams, their width,(i.e. the dimension of individual beams along the direction of movementof the material being treated) and their average spacing, and also uponthe linear speed and reaction characteristics of the system beingtreated. For a free-radical reaction with kinetics correspondingapproximately to bimolecular termination the main reactioncharacteristic involved is the radical halfllife although this time alsodepends upon the irradiation dose-rate.

Such equipment may consist of a number (preferably greater than four) ofseparate electron sources suitably spaced in accordance with theprinciples stated below, but a preferred equipment consists of one ormore special irradiation sources each comprising four or more electronbeams preferably having in common one or more of the following: anodeplate, vacuum chamber and/or pumping system, high voltage power supply,filament supply and/or biassing supply, control instrumentation,interlocks etc. The several electron beams may each have its ownassociated source of electrons and beam forming system or several beamsmay originate from a single source of electrons with suitable beamsplitting or scanning systems. The accelerating voltage should be as lowas possible for reasons of economy, but must be such as to givereasonably uniform penetration within the material to be treated.

We are aware that it has been proposed to subject materials to electronradiation from a main generator of substantial voltage and current, andthen to subject it later to the actions of one or more auxiliary sourcesof lower electron energy and lower density but this has been solely toachieve economy by approaching greater uniformity of dosage measuredperpendicularly in from the surface of the material, as compared with asingle source, and this prior proposal was wholly dependent on thepenetration of the electrons from the auxiliary generator being lessthan the penetration of those from the main generator. It has also beenproposed, again for reasons of economy, to irradiate strip material asit wound up onto a reel, which is rotated under the electron beam, sothat each particle is irradiated repeatedly, but again the energy andpenetration of the electrons are different each time.

The invention will now be further described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a graph to illustrate the thinking behind the invention;

FIG. 2 is a diagrammatic side elevation of the irradiation apparatus,shown in section; and

FIG. 3 is a transverse section to a larger scale through one of thewindows in the anode, showing the details of the window construction.

Referring first to FIG. 1, this is a graph showing the intensity I ofradiation impinging on an element of material being irradiated, and theradical concentration produced, both against time. An efi'icient form ofirradiation for certain reactions, especially polymerization andgrafting reactions, would be a relatively low dose-rate maintained for along time, as indicated by the curves A. The total dosage received bythe material is given by the area under the intensity curve. Now if thesame total dosage were given in a very much shorter time (but with aconsequently higher intensity) as indicated by the curve B, theefi'ectiveness of the reaction obtained would be very much less. Thereason for this is as follows: The impingement of the electrons on themolecules of the material serves to create radicals, which initiateactive (i.e. free radical) polymerized chains, until tenninated byreaction with another active chain. At higher free-radicalconcentrations, the polymerization chains are shorter and therefore alower degree of conversion or cure is achieved for a given amount ofenergy.

Therefore it is desirable to spread the radiation over a period of timelonger than the radical half-life. As indicated earlier, it is difficultand expensive to produce a uniform dosage over a long period, but thesame result, with the same advantages, is approached asymptotically bythe use of several sources irradiating the material in turn. The curvesC show radiation produced by six separate sources each having a widthonly a fraction of the spacing between the sources, and the resultingradical concentration which is lower than for case B and approximates tothe more efficient case A.

Referring now to FIG. 2, the apparatus comprises an evacuated chamber Ccontaining a number (five in the example illustrated) of individualelectron sources spaced at substantially uniform intervals along itslength and shown as filaments F with biassing grids C. Beam-formingsystems (not shown) associated with each filament form verticallydownwardly directed beams which pass through five respective windows Win a single final anode A which is common to all five electron sources.The anode A forms the bottom wall of the chamber C and is preferably atearth potential, the filaments F being therefore at a high negativepotential, to simplify external insulation problems.

The five beams emerging from the final anode A at substantially equallyspaced intervals along a straight line each have a width (measured alongthat line) approximately defined by the associated window. They allimpinge on a strip of (flat) material M, the object to be irradiated,which has its plane substantially perpendicular to the axes of the beamsand is moving in a direction parallel to the line along which the fivebeams are arrayed.

To maintain the vacuum inside the chamber C each of the windows W musthave a membrane or foil T across it, as shown in FIG. 3. The foil is asthin as possible commensurate with the necessary strength, and in oneconstruction is clamped by means of a clamping ring R with inner andouter O-ring seals S and with the annular space between the seals pumpedby connection to an auxiliary pump (not shown). The heat generated inthe foil by the impingement of the electrons is carried away by coolingwater circulating in a duct D.

The following table gives a list of electron energies (measured as theaccelerating voltage between filament and final anode) necessary toirradiate materials of different thicknesses, and of the thickness ofthe foil membrane that is suitable with this voltage, the membrancebeing of aluminum foil;

Approximate Maximum Instead of aluminum, the foil could be of any othersuitable material, such as beryllium or an aluminum/magnesium alloy, ortitanium. Where the electron energy is low, say below 200 KV, and whereconsequently it is necessary to use very thin foils to avoidunacceptably high losses in the window, the clamping ring or frame Rhelps to ensure good thermal contact with the anode A but still allowsthe foil to be replaced when necessary without undue difficulty.

The main parameters of the apparatus which determine the total doserequired for a given material are the overall length L of theirradiation zone and the number of beams N. The ratio of the doserequired using the apparatus of the invention to the dose that would berequired from an orthodox single-source narrow-beam apparatus alsodepends on the speed of travel V of the material and on thecharacteristics of the material, namely its radical life-time t and thedependence of the dose R on the dose-rate l. The overall relationshipsare complex, but the following two examples illustrate the factorsinfluencing the design of the apparatus and of the sources used in it.Both the examples refer to reaction systems in which R is proportionalto the square root of 1.

EXAMPLE A Where the overall length L of the irradiation zone can bevaried and the spacing between the individual beams is large comparedwith, or at least much greater than, the distance Vt moved by thematerial during the radical life-time, then the radiation dose R variesapproximately inversely with the number of beams; thus if we double thenumber of beams without decreasing their spacing we halve the totaldosage that is required to carry out the given reaction. Secondly thedosage required is approximately proportional to the velocity V ofmovement of the material. Thus doubling the speed of movement will meanthat double the total dosage is required, so that the intensity ofradiation must be multiplied by four.

EXAMPLE B Where the overall length of the irradiation zone is fixed,then increasing the number of beams while reducing the spacing betweenthem will reduce the total dosage required. The dosage will fallasymptotically to that dosage which would be required if the source werea uniform one of length L. Where the overall time of irradiation isapproximately greater than the radical life-time, the dose requiredusually falls within about percent of the dose required for uniformirradiation after some 10 to sources. Where the number of sources is ofthis order, i.e. is such that the dosage required is not substantiallydifferent from that which would be required from a uniform source oflength L, the dosage R is approximately proportional to the velocity Vand inversely proportional to the length L.

Although the apparatus according to the invention allows reactions thatrequire a low dose-rate to be performed at appreciably lower doses thanwith normal single-source electron radiation apparatus, it may also beusefully employed in reactions that do not necessarily require a lowdose rate, for it has other advantages such as low secondary heating and(particularly at low voltages) simplified electron windows.

lclaim:

1. Apparatus for the continuous treatment of polymeric materials withelectron radiation to cause chemical changes therein comprising at leastfive electron beam sources spaced apart along a straight line, saidsources producing beams which are mutually parallel and are directedtransverse to said line, a single common anode plate extending parallelto said line and placed such that said beams are directed substantiallyperpendicular to said anode plate, a number of windows in said anodeplate corresponding to the number of said sources, each one of saidwindows being associated with one of said beams and permitting saidassociated beam to pass through said anode plate, the width of each ofsaid windows in a direction parallel to said line being less than thespacing between adjacent ones of said windows, and means for movingmaterial past said anode plate through the paths of said beams insuccession in a direction parallel to said line whereby any givenelement of said material receives successive part-doses of electronradiation from each of said beams in turn.

1. Apparatus for the continuous treatment of polYmeric materials withelectron radiation to cause chemical changes therein comprising at leastfive electron beam sources spaced apart along a straight line, saidsources producing beams which are mutually parallel and are directedtransverse to said line, a single common anode plate extending parallelto said line and placed such that said beams are directed substantiallyperpendicular to said anode plate, a number of windows in said anodeplate corresponding to the number of said sources, each one of saidwindows being associated with one of said beams and permitting saidassociated beam to pass through said anode plate, the width of each ofsaid windows in a direction parallel to said line being less than thespacing between adjacent ones of said windows, and means for movingmaterial past said anode plate through the paths of said beams insuccession in a direction parallel to said line whereby any givenelement of said material receives successive part-doses of electronradiation from each of said beams in turn.